In many industries such as semiconductor manufacturing the characterization of surface structures comprise an important step in verifying the integrity of the manufacturing process. These structures include critical dimensions (CD's), depth, profile, etc. One method of characterizing structures is to use reflectance spectrophotometry.
Reflectance spectrophotometry is a technique where a beam of light is directed toward a target. The light reflects off of the target and is collected in a spectrophotometer. When structures are arranged in a repeating pattern, even if the structures are non-symmetrical, evidence of the structure pattern shows up in the reflected light. By analyzing the properties of the collected light and comparing them to the properties of the original light source, properties of the structures, such as those used in diffraction gratings for example, can be determined.
FIG. 1 shows an example of the disclosure of U.S. Pat. No. 5,991,022 by Buermann, Forouhi, and Mandella, which describes a spectrophotometric apparatus with toroidal mirrors with the desired characteristics stated above. A light source 102 produces a beam of light 104. The beam 104 strikes mirrors 106 A, B in route to illuminating the substrate 108. Light reflected off of the substrate 108 is directed by mirrors 106 C, D into the photodetector 110, which is a spectrophotometer. Data from the photodetector 110 is sent to a computer 112 for processing. The angle of incidence and angle of reflection are shown with θi and θr respectively.
FIG. 2 shows the path of travel for light beam for the system shown in FIG. 1. This is a side view. The light beam 104 comes in from the left side of the page. The light beam 104 strikes the mirror 106B, and is reflected toward the substrate 108. For illustrative purposes only, the substrate 108 and the cross-sectional areas 202, 204 of the light beam 104 are shown in a slight isometric configuration. The cross-sectional area 202 of the beam 104 is not reduced as the beam 104 reflects off of the mirror 106B. Likewise, the cross-sectional area 204 of the beam 104 as the beam 104 strikes the substrate 108 is not reduced. Thus, in this prior art example, the beam 104 travels in its entirety from the light source 102 to the substrate 108.
FIG. 3 shows the path of travel for a light beam for the system shown in FIG. 1. This is a front view, which looks at the system from the left side of FIG. 2. Again, for illustrative purposes only, the cross sectional areas 202, 204 and the substrate 108 are shown in a slight isometric configuration. In this view, the beam 104 approaches the mirror 106B from above the page. The beam 104 reflects off of the mirror 106B toward the substrate 108. The beam 104 is not reduced as it travels from the light source 102 to the substrate 108.
It should also be apparent from the prior art system shown in FIG. 1 that the beam 104 that reflects off of the substrate 108 is not reduced in its cross-sectional area before it reaches the photodetector 110.
In an apparatus used to characterize structures using reflectance spectrophotometry, it is desirable that light reflected from the material is directed into a spectrophotometer by an optical relay that has a minimum of aberrations. First, it is desirable to eliminate the chromatic aberrations to achieve an accurate measurement. However, lenses and mirrors have other, nonchromatic aberrations as well. These aberrations include spherical aberration, coma, astigmatism, curvature of field, and distortion. All lenses and mirrors suffer from these aberrations to some extent, even if they are perfectly machined. The existence of these aberrations represents a fundamental limitation on the nature of a lens or mirror—a limitation that is generally neglected in the paraxial approximation of introductory texts. Since the structures of interest often are patterned structures, such as integrated circuits, diffraction gratings, or contact holes, the structures usually are small and the areas that they are comprised of are small. Consequently, the measurement area is desirably small enough to fit within the entire pattern, yet large enough so that there are repeating structures in the measurement area. Thus, it is desirable that a reflectance spectrophotometric apparatus be able to image a small area, on the order of 50 microns in diameter, of the area of interest to a spectrophotometer with as little aberration as possible. It is also desirable that the apparatus include hardware for translating the target with respect to the imaging optics so that different regions of the target may be characterized.
One disadvantage for systems that use larger angles of incidence is that they do not correctly measure trenches with high aspect ratios (i.e. deep and narrow). With these systems it is possible that the incoming light will not reach the bottom of the trench before striking a wall. This effect is sometimes referred to as “shadowing.” In order to obtain an accurate measurement, is it desirable that the beam strikes the bottom of the trench and reflect out of the trench without hitting the side walls of the trench.
Another disadvantage for systems that use larger angles of incidence is that they take more time to determine structure geometries when the trenches of the geometry in question are parallel to the plane of the angle of incidence. In order to speed the calculation time, smaller angles of incidence can be used.
Thus, it is desirable for a reflective spectrophotometric device to have an angle of incidence that is small.